Memory tags in the form of Radio Frequency Identification (RFID) tags are well known in the prior art, and the technology is well established (see for example: RFID Handbook, Klaus Finkenzeller, 1999, John Wiley & Sons). RFID tags come in many forms but all comprise an integrated circuit with information stored on it and a coil which enables it to be interrogated by a read/write device generally referred to as a reader. Until recently RFID tags have been quite large, due to the frequency they operate at (13.56 MHz) and the size of coil they thus require, and have had very small storage capacities. Such RFID tags have tended to be used in quite simple applications, such as for file tracking within offices or in place of or in addition to bar codes for product identification and supply chain management.
Much smaller RFID tags have also been developed, operating at various frequencies. For example Hitachi-Maxell have developed “coil-on-chip” technology in which the coil required for the inductive link is on the chip rather than attached to it. This results in a memory tag in the form of a chip of 2.5 mm square, which operates at 13.56 MHz. In addition Hitachi has developed a memory tag referred to as a “mu-chip” which is a chip of 0.4 mm square and operates at 2.45 GHz. These smaller memory tags can be used in a variety of different applications. Some are even available for the tagging of pets by implantation.
Although it is known to provide tags with their own power source, in many applications the tag is also powered by the radio frequency signal generated by the reader. Such a known system is shown in FIG. 1 where a reader is indicated generally at 10 and a tag at 12. The reader 10 comprises a radio frequency generator 13 and a resonant circuit part 11, in the present example comprising an inductor 14 and a capacitor 15 connected in parallel. The inductor 14 comprises a antenna. The resonant circuit part will have a particular resonant frequency in accordance with the capacitance and inductance of the capacitor 15 and the inductor 14, and the frequency generator 13 is operated to generate a signal at that resonant frequency.
The tag 12 similarly comprises a resonant circuit part generally illustrated at 16, a rectifying circuit part generally indicated at 17 and a memory 18. The resonant circuit part 16 comprises an inductor 19 which again comprises in this example a loop antenna, and a capacitor 20. The resonant circuit part 16 will thus have a resonant frequency set by the inductor 19 and capacitor 20. The resonant frequency of the resonant circuit part 16 is selected to be the same as that of the reader 10. The rectifying part comprises a forward-biased diode 21 and a capacitor 22 and thus effectively acts as a half-ware rectifier.
When the reader 10 and the tag 12 are sufficiently close, a signal generated by the frequency generator 13 will cause the resonant circuit part 11 to generate a reader signal comprising a high frequency electromagnetic field. When the resonant circuit part 16 is moved within this field, a current will be caused to flow in the resonant circuit part 16, drawing power from the time varying magnetic field generated by the reader. The rectifying circuit part 17 will then serve to smooth the voltage across the resonant frequency part and provide a power supply storage. The rectifying circuit part 17 is sufficient to supply a sufficiently stable voltage to the memory 18 for the memory to operate.
It is possible however, when no tag 12 is sufficiently close to the reader 10, the electromagnetic field generated by the reader 10 could be coupled to other objects, particularly objects containing metal, such as a glass frame a pen or such wires as may be found on a desk. This may be undesirable. It is possible in such circumstances, the reader would not meet prescribed legal regulations or guidelines relating to the level of radiated power from radio transmitters. However, it will be apparent that simply reducing the power of the signal transmitted by the reader will reduce both the range at which the reader may operate and the power available for operation of the tag 32. An aim of the present invention is to reduce or overcome the above problem.